In situ bonding of carbon fibers and nanotubes to polymer matrices

ABSTRACT

A method for forming a carbon fiber-reinforced polymer matrix composite by distributing carbon fibers or nanotubes into a molten polymer phase comprising one or more molten polymers; and applying a succession of shear strain events to the molten polymer phase so that the molten polymer phase breaks the carbon fibers successively with each event, producing reactive edges on the broken carbon fibers that react with and cross-link the one or more polymers. The composite shows improvements in mechanical properties, such as stiffness, strength and impact energy absorption.

TECHNICAL FIELD

The present invention relates to high efficiency mixing methods totransform a polymer composite containing carbon fibers. The presentinvention also relates to methods to activate carbon fibers andnanotubes using in situ mechanical breakage or cutting of the fibers ornanotubes in the presence of a molten polymer.

BACKGROUND

Polymer compositions are being increasingly used in a wide range ofareas that have traditionally employed the use of other materials, suchas metals. Polymers possess a number of desirable physical properties,are light weight, and inexpensive. In addition, many polymer materialsmay be formed into a number of various shapes and forms and exhibitsignificant flexibility in the forms that they assume, and may be usedas coatings, dispersions, extrusion and molding resins, pastes, powders,and the like.

There are various applications for which it would be desirable to usepolymer compositions, which require materials with electricalconductivity. However, a significant number of polymeric materials failto be intrinsically electrically or thermally conductive enough for manyof these applications.

Most composites are made with the understanding that there will be onlyweak secondary bonds that exist between the fibers and polymer. Thismakes it necessary for very high aspect ratios of fibers to be used inorder to get reasonable stress transfer, or else the fibers will slipupon load.

Some commercial applications of carbon fiber-reinforced polymer matrixcomposites (CF-PMCs) include aircraft and aerospace systems, automotivesystems and vehicles, electronics, government defense/security, pressurevessels, and reactor chambers, among others.

Progress in the development of low cost methods to effectively producecarbon fiber-reinforced polymer matrix composites (CF-PMCs) remains veryslow. Currently, some of the challenges that exist affecting thedevelopment of CF-PMCs viable for use in real world applications includethe expense of the materials and the impracticality of the presentlyused chemical and/or mechanical manipulations for large-scale commercialproduction. It would thus be desirable for a low cost method to producea CF-PMC suitable for large-scale commercial production that offers manyproperty advantages, including increased specific stiffness andstrength, enhanced electrical/thermal conductivity, and retention ofoptical transparency.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to the discovery that strongerprimary bonds between carbon fibers and carbon based polymers can becreated, making it possible to get very high stress transfer with muchshorter fibers in the resulting composites. Thus the disclosure providesstiffer and stronger polymer-carbon fiber composites and methods forforming them. A variety of carbon fibers are useful in the methods,including single or multi-walled carbon nanotubes (SWCNTs and MWCNTs,respectively), carbon nanofibers and standard micron-sized carbonfibers. The method works well in conjunction with a variety of polymersthat possess chemical groups having one or more double bonds(carbon-carbon double bonds, carbon-oxygen double bonds, etc.) orchemical groups having one or more tertiary carbons, viz.,

The fibers are broken in the presence of molten polymers during meltprocessing. Fiber breakage can be accomplished either by having aspecially designed cutting tool in the melt processing equipment, orthrough high shear during melt processing, or by a combination of thetwo. The opening up of new fiber ends by breaking the fibers whilesurrounded by liquid polymers introduces dangling bonds, or reactivefree radicals, on the fiber ends that represent sites for strong bondingby the polymers with the attributes mentioned above. The resulting solidcomposites have improved mechanical properties upon cooling, withoptimal fiber length, and, consequently, cost can be greatly reduced bythis bonding within the composite.

One aspect of the invention is directed to a method for forming a carbonfiber-reinforced polymer matrix composite, comprising: (a) distributingcarbon fibers into a molten carbon-containing polymer phase comprisingone or more molten carbon-containing polymers; (b) breaking said carbonfibers in the presence of said molten thermoplastic polymer phase by (i)applying a succession of shear strain events to the molten polymer phaseso that said molten polymer phase breaks said carbon fibers, or (ii)mechanically cutting said carbon fibers, thereby producing reactiveedges that react with and cross-link said one or more carbon-containingpolymers; and (c) thoroughly mixing said broken or cut carbon fiberswith said molten polymer phase. In one embodiment at least one of saidone or more carbon-containing polymers contains chemical groups havingone or more double bonds or one or more tertiary carbons. In anotherembodiment said molten carbon-containing polymer phase comprises anylon, which can be nylon 66. In one embodiment of the method saidcarbon fibers are selected from the group consisting of single-walledcarbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs),carbon nanofibers, and micron-sized carbon fibers.

Another aspect of the invention is directed to a method for forming ahigh-strength carbon fiber-reinforced polymer matrix composite,comprising: (a) forming the composite described above into cross-linkedpolymer particles; and (b) distributing said polymer particles into anon-cross-linked molten host matrix polymer.

Another aspect of the invention is directed to a carbon fiber-reinforcedpolymer matrix composite prepared according to the method describedabove. In one embodiment said polymer is nylon 66. Another aspect of theinvention is directed to a high strength carbon fiber-reinforced polymermatrix composite prepared according to the above method.

In some embodiments the composite shows improved stiffness and strengthversus a composite lacking covalent bonding between carbon fibers andpolymer. In some embodiments the composite shows improved impact energyabsorption versus a composite lacking covalent bonding between carbonfibers and polymer.

A further aspect of the invention is directed to a polymer compositecomprising polymer chains inter-molecularly cross-linked by brokencarbon fibers having carbon atoms with reactive bonding sites on thebroken edges of said fibers. In one embodiment an automotive, aircraftor aerospace part is formed from the composite. In one embodiment saidpart is an engine part. Another aspect of the invention is directed tocarbon fiber cross-linked polymer particles formed from the abovecomposite. Another aspect is directed to a polymer compositioncomprising a host thermoplastic polymer and the carbon fibercross-linked polymer particles disclosed above. Automotive, aircraft oraerospace part can also be formed from such a polymer composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays a series of scanning electron microscopy (SEM) images,arranged from low-to-high magnification, showing that a typicalas-synthesized particle consists of loosely-agglomerated multi-wall CNTs(MWCNTs).

FIG. 2 displays DSC heating-cooling-heating curves for Nylon 66,indicating 267° C. melting point, 225° C. freezing point and a glasstransition temperature of about 60° C.

FIG. 3 shows stress-sweep curves for Nylon 66, indicating that theLinear Viscosity Region (LVR) is around 0.4% strain.

FIG. 4 displays frequency-sweep curves for Nylon 66 at 0.4% strain.

FIG. 5 displays frequency-sweep curves for Nylon 66, indicating that theviscosity decreases with increasing shear rate (viscosity vs. shearrate).

FIGS. 6(a) through 6(d) display a series of SEM images arranged fromlow-to-high magnification, providing evidence for pull-out of CNTs,after cryogenic fracture of an 8% CNT-reinforced Nylon composite.

FIGS. 7(a) and 7(b) display transmission electron microscopy (TEM)images. FIG. 7(a) provides evidence for adhesive bonding between a MWCNTand an amorphous or non-crystalline Nylon 66 matrix; that is, nocovalent bonding is observed for a MWCNT having no broken ends in aNylon 66 matrix. In contrast, FIG. 7(b) shows crystallization of denserNylon 66 (less transparent to the electron beam) at or near thefractured end of a MWCNT.

FIGS. 8(a) through 8(f) display DSC heating-cooling-heating curves forNylon 66 composites of the invention containing 1% to 6% CNT,respectively.

FIGS. 9(a) through 9(c) display TEM images of a MWCNT in a Nylon 66matrix with freshly broken ends where bonding is promoted. The picturesshow a high density of polymer at the broken ends of the CNTs,indicating covalent bonding between the nanotubes and the Nylon 66polymer.

FIG. 10 shows an SEM micrograph of 30 wt. % carbon fiber (CF) in PEEKprepared using high shear melt-processing of continuous CF cut to 1 mlengths and PEEK.

FIGS. 11(a) through (d) show the mechanical properties of CF-reinforcedPEEK as a function of increasing CF concentration prepared using highshear melt-processing of continuous CF cut to 1 m lengths and PEEK asfollows: (a) flexural stress-strain curves; (b) flexural modulus; (c)flexural strength; and (d) Izod impact resistance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This disclosure is not limited to the particular systems, methodologiesor protocols described, as these may vary. The terminology used in thisdescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

As used in this document, the singular forms “a,” “an,” and “the”include plural reference unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. All publications mentioned in this document are incorporatedby reference. All sizes recited in this document are by way of exampleonly, and the invention is not limited to structures having the specificsizes or dimensions recited below. Nothing in this document is to beconstrued as an admission that the embodiments described in thisdocument are not entitled to antedate such disclosure by virtue of priorinvention. As used herein, the term “comprising” means “including, butnot limited to.”

One aspect of the present invention is directed to mechanicalfunctionalization of carbon fibers processed in situ with moltenpolymers to create reactive bonding sites at the ends of the fibers. Thereactive sites react with the polymer to chemically bond the carbonfibers to the polymer.

This can be done with a variety of carbon fibers, including single ormulti-walled carbon nanotubes and standard micron sized carbon fibers.It works well in conjunction with a variety of polymers that possesschemical groups having double bonds (carbon-carbon double bonds,carbon-oxygen double bonds, etc.) or various tertiary carbon bonds.Similar observations of good bonding at sites of broken covalentgraphite and graphene bonds have been made while mechanicallyexfoliating graphite into graphene in situ with polymers.

The fibers are broken or cut while in molten polymers during meltprocessing, and this can be done either by having a specially designedcutting tool in the melt processing equipment, or through high shear inthe melt processing, or by a combination of the two. The opening up ofnew fiber ends by breaking or cutting the fibers while surrounded byliquid polymers introduces dangling bonds having unfilled valencies(free radicals) which provide reactive sites on the fiber ends, whichrepresent sites for strong bonding, such as covalent bonding, by thepolymers having the attributes mentioned above. The resulting solidcomposites have improved mechanical properties upon cooling, and theoptimal fiber length, and, subsequently, cost will be greatly reduced bythis bonding.

The following term(s) shall have, for purposes of this application, therespective meanings set forth below:

The term “polyetherketone” (PEK) denotes polymers characterized by amolecular backbone having alternating ketone and ether functionalities.The most common PEK are polyaryl (PAEK) which contain an aryl or phenylgroup linked in the 1- and 4-positions between the functional groups.The very rigid backbone gives such polymers very high glass transitionand melting temperatures compared to other plastics. The most common ofthese high-temperature resistant materials is polyetheretherketone(PEEK). Other representatives of polyetherketones include PEKK(poly(etherketoneketone)), PEEEK (poly(etheretheretherketone)), PEEKK(poly(etheretherketoneketone)) and PEKEKK(poly(etherketone-etherketoneketone)).

In one aspect, the present invention provides a high efficiency mixingmethod to transform a polymer composite that contains carbon fibers intobroken carbon fibers having reactive ends or edges, by compounding in abatch mixer or extruder that imparts repetitive, high shear strainrates. The method is low cost to produce a CF-PMC that offers numerousproperty advantages, including increased specific stiffness andstrength, enhanced electrical/thermal conductivity, and retention ofoptical transparency. Furthermore, these properties are tunable bymodification of the process, vide infra. In some cases, an inert gas orvacuum may be used during processing. Other advantages of in situ carbonfiber breaking are that it avoids handling size reduced carbon fibers,and also avoids the need to disperse them uniformly in the polymermatrix phase. Superior mixing produces finer composite structures andvery good particle distribution.

It should be understood that essentially any polymer inert to carbonfibers or nanotubes and capable of imparting sufficient shear strain toachieve the desired carbon fiber breakage may be used in the method ofthe present invention. Examples of such polymers include, withoutlimitation, poly-etheretherketones (PEEK), polyetherketones (PEK),polyphenylene sulfides (PPS), polyethylene sulfide (PES),polyetherimides (PEI), polyvinylidene fluoride (PVDF), polysulfones(PSU), polycarbonates (PC), polyphenylene ethers, aromatic thermoplasticpolyesters, aromatic polysulfones, thermoplastic polyimides, liquidcrystal polymers, thermosplastic elastomers, polyethylene,poly-propylene, polystyrene (PS), acrylics, such aspolymethylmethacrylate (PMMA), polyacrylo-nitrile (PAN), acrylonitrilebutadiene styrene (ABS), and the like, ultra-high-molecular-weightpolyethylene (UHMWPE), polytetrafluoroethylene (PTFE/Teflon®),polyamides (PA) such as nylons, polyphenylene oxide (PPO),polyoxymethylene plastic (POM/Acetal), polyarylether-ketones,polyvinylchloride (PVC), mixtures thereof and the like. Polymers capableof wetting the carbon fiber surface may be used as well as high meltingpoint, amorphous polymers in accordance with the method of the presentinvention.

Carbon fiber-reinforced polymers according to the present inventiontypically contain between about 0.1 and about 30 wt % carbon fibers ornanotubes. More typically, the polymers contain between about 1.0 andabout 10 wt % carbon fibers or nanotubes. According to one embodiment,the carbon fiber-reinforced polymer matrix composite contains from 1 wt% to 10 wt %, or from 2 wt % to 9 wt %, or from 3 wt % to 8 wt %, orfrom 4 wt % to 7 wt %, or from 5 wt % to 6 wt % carbon fibers ornanotubes (based on the total composite weight). Polymer masterbatchestypically contain up to about 65 wt % carbon fibers or nanotubes, andmore typically between about 5 and about 50 wt % carbon fibers ornanotubes. According to one embodiment, the masterbatches containbetween about 10 and about 30 wt % carbon fibers or nanotubes.

Mechanical functionalization of carbon fibers within a polymer matrixmay be accomplished by a polymer processing technique that impartsrepetitive high shear strain events to mechanically break the carbonfibers within the polymer matrix.

A succession of shear strain events is defined as subjecting the moltenpolymer to an alternating series of higher and lower shear strain ratesover essentially the same time intervals so that a pulsating series ofhigher and lower shear forces associated with the shear strain rate areapplied to the carbon fibers in the molten polymer. Higher and lowershear strain rates are defined as a first higher, shear strain rate thatis at least twice the magnitude of a second lower shear strain rate. Thefirst shear strain rate will range between 100 and 10,000 sec⁻¹. Atleast 1,000 to over 10,000,000 alternating pulses of higher and lowershear strain pulses are applied to the molten polymer in order to breakthe carbon fibers.

After high-shear mixing, the mechanically size reduced carbon fibers areuniformly dispersed in the molten polymer, are randomly oriented, andhave high aspect ratio.

In one embodiment, graphite microparticles are also added to the moltenpolymer and are mechanically exfoliated into graphene via the successionof shear strain events. Graphite microparticles are generally no greaterthan 1,000 microns in size, and the extent of exfoliation of thegraphite microparticles can generally be from 1 to 100%, resulting in agraphene to graphite weight ratio ranging from 1:99 to 100:0. Such anexfoliation method is disclosed in US 2015/0267030, the entiredisclosure of which is incorporated herein by reference.

The amount of graphite added to the molten polymer can be an amount upto and including the amount of carbon fibers and nanotubes added,provided that the total content of carbon fibers, nanotubes andresulting graphene or mixture of graphite and graphene does not exceed65 wt %. Typically, the weight ratio of graphene, or a mixture ofgraphite and graphene, to carbon fibers and/or nanotubes ranges between5:95 and 50:50, and more typically between 25:75 and 33:67.

In one embodiment, the extrusion compounding elements are as describedin U.S. Pat. No. 6,962,431, the disclosure of which is incorporatedherein by reference, with compounding sections, known as axial flutedextensional mixing elements or spiral fluted extensional mixingelements. The compounding sections act to elongate the flow of thepolymer and carbon fibers, followed by repeated folding and stretchingof the material. This results in superior distributive mixing, which inturn, causes progressive breakage of the carbon fibers. Batch mixers mayalso be equipped with equivalent mixing elements. In another embodiment,a standard-type injection molding machine is modified to replace thestandard screw with a compounding screw for the purpose of compoundingmaterials as the composition is injection molded. Such a device isdisclosed in US 2013/0072627, the entire disclosure of which isincorporated herein by reference.

Automated extrusion systems are available to subject the compositematerial to as many passes as desired, with mixing elements as describedin U.S. Pat. No. 6,962,431, and equipped with a re-circulating stream todirect the flow back to the extruder input. Since processing of thecarbon fiber-reinforced polymer is direct and involves minimal handlingof carbon fibers, fabrication costs are low.

The shear strain rate within the polymer is controlled by the type ofpolymer and the processing parameters, including the geometry of themixer, processing temperature, and speed in revolutions per minute(RPM).

The required processing temperature and speed (RPM) for a particularpolymer is determinable from polymer rheology data given that, at aconstant temperature, the shear strain rate ({dot over (γ)}) is linearlydependent upon RPM, as shown by Equation 1. The geometry of the mixerappears as the rotor radius, r, and the space between the rotor and thebarrel, Δr.

$\begin{matrix}{\overset{.}{\gamma} = {\left( \frac{2\pi\; r}{\Delta\; r} \right)\left( \frac{RPM}{60} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Polymer rheology data collected for a particular polymer at threedifferent temperatures provides a log shear stress versus log shearstrain rate graph.

Examples of host polymers include, but are not limited to,polyetheretherketone (PEEK), polyetherketone (PEK), polyphenylenesulfide (PPS), polyethylene sulfide (PES), polyether-imide (PEI),polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC),polyphenylene ether, aromatic thermoplastic polyesters, aromaticpolysulfones, thermo-plastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylene, polypropylene, polystyrene (PS),acrylics such as polymethylmethacrylate (PMMA), polyacrylonitrile (PAN),acrylonitrile butadiene styrene (ABS), and the like,ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE/Teflon®), polyamides (PA) such as nylons,polyphenylene oxide (PPO), polyoxymethylene plastic (POM/Acetal),polyimides, polyarylether-ketones, polyvinylchloride (PVC), acrylics,mixtures thereof and the like. When the host polymer and thecross-linked polymer are the same polymer species, the cross-linkedpolymer particles are essentially a concentrated masterbatch of thedegree of cross-linked species desired to be introduced to the polymerformulation.

Processing and Characteristics of CNT-Reinforced Nylon 66 Composites

A polymer-carbon nanotube composite (PCNC) differs from a conventionalcarbon-fiber composite in that there is a much higher interface areabetween reinforcing carbon and polymer matrix phases. It has beenproposed that introducing a uniform distribution of carbon nanotubes(CNTs) into a polymer matrix should yield property enhancements that gobeyond that of a simple rule of mixtures. The challenge is to take fulladvantage of the exceptional properties of CNTs in the compositematerial.

Carbon nanotubes are considered to be ideal reinforcing material forpolymer matrices because of their high aspect ratio, low density,remarkable mechanical properties, and good electrical/thermalconductivity. One of the matrices that has been studied is commerciallyimportant Nylon 66. However, property improvements have not beensignificant to date, apparently due to poor interfacial CNT/polymerbonding and severe CNT agglomeration.

These obstacles have now been overcome by utilizing a new processingroute that involves high-shear mixing in a molten polymer to inducede-agglomeration and dispersal of CNTs, while enhancing adhesive bondingand covalent bonding by creating new sites on the CNTs to which thepolymer chains can bond. An attempt is also being made to increaseimpact energy absorption by forming a biphasic composite, comprising ahigh fraction of strong CNT-reinforced Nylon particles uniformlydispersed in a tough Nylon matrix.

A carbon nanotube (CNT) consists of a sheet of hexagonal-bonded carbonatoms rolled up to form a tube. A single-walled carbon nanotube (SWCNT)comprises a single layer of this tubular structure of carbon atoms.However, the structure of a multi walled carbon nanotube (MWCNT) isstill open to some debate. In one model, a MWCNT is imagined to be asingle graphene sheet rolled up into a scroll. In another model, a MWCNTis considered to be made of co-axial layers of helically-aligned carbonhexagons, with matching at the joint lines, leading to a nested-shellstructure. In yet another model, a combination of scroll-like andnested-shell structures has been proposed.

It is known that increases in elastic modulus and strength of Nylon-CNTcomposite resulted from making small additions of CNTs to polymermatrices. While Van der Waals bonding dominates interactions betweenCNTs and polymers, adhesion in some CNT composites also occurs viacovalent bonds, which has been shown to play a role in reinforcement ofCNT composites.

Measurements by AFM of the pull-out force necessary to remove a givenlength of an individual MWCNT embedded in polyethylene-butene copolymerhas demonstrated covalent bonding between the outer layer of a MWCNT andthe polymer matrix. It also showed that the polymer matrix in the nearvicinity to the interface behaved differently than the polymer in thebulk, which is attributed to the outer diameter of a CNT having the samemagnitude as the radius of gyration of the polymer chain.

Because of the tendency of CNTs to agglomerate, difficulty of aligningthem in the matrix and often poor load transfer, there have been anumber of reported attempts to produce composites using differentpolymer matrix phases.

The present invention provides remarkable improvements in stiffness andstrength of a CNT-reinforced Nylon composite, vide infra. The compositesare characterized by an increase in impact energy absorption. Processingparameters which achieve superior mechanical properties and performanceare provided herein.

EXAMPLES

The present invention is further illustrated by the following examples,which should not be construed as limiting in any way.

Modified Randcastle Extrusion System Small Scale Extension Mixer

The design of the existing small batch mixer may be modified to providehigher shear rate, which in turn provides superior mechanical breakageof the carbon fibers within the polymer matrix. The shear rate, {dotover (γ)}, is calculated according to Equation 1, where r is the toolingradius and Δr is the clearance for compounding. Machine modificationsare listed in the Table below, along with the maximum achievable shearrate. The newly designed mixer has a maximum shear rate 22 times that ofthe current mixer, which will provide enhanced mechanical breakage ofcarbon fibers within a polymer matrix at shorter lengths of time. Inother words, the crystal size, D, may be reduced to smaller dimensionsin a more efficient length of time.

TABLE Modifications of the Randcastle Extrusion System's Small ScaleExtension Mixer to provide enhanced mechanical functionalization ofcarbon fibers Current Improved Randcastle Randcastle Mixer Mixer ToolingRadius (inches) 0.5 1 Clearance for Compounding, Δr (in) 0.04 0.01Maximum RPM 100 360 Maximum Shear Strain Rate (sec⁻¹) 133 2900

Modified Single Screw Extrusion

Randcastle has made modifications to the extruder screw that will betterenable mechanical breakage of carbon fibers in a polymer matrix tofabricate a CF-PMC.

Example 1 Materials and Processing Parameters

Well-characterized MWCNT powder, with particle size in the 10-50 μmrange, was acquired from CNano Technology. A sequence of back-scatteredSEM micrographs, FIG. 1, shows that a typical particle consists ofloosely-agglomerated multi-wall CNTs (MWCNTs), most of which are about30-40 nm in diameter and >1 μm in length, i.e. have high aspect ratios.In several cases, the CNTs have white-contrasting tips, which aretransition-metal catalyst particles. Hence, it is apparent that the CNTsare produced by the particle-at-the-tip growth mechanism.

Pelletized Nylon 66, with pellet size in the 1-5 mm range, was acquiredfrom Dupont Inc. A differential scanning calorimetry (DSC) curve, FIG.2, shows that the melting and freezing temperatures are 267° C. and 225°C., respectively; glass transition temperature is about 60° C. SinceNylon 66 readily adsorbs water upon exposure to ambient air, as-receivedand processed powders are vacuum-dried at 85° C. for 24 hours beforefurther processing.

A laboratory-scale high-shear mixer with 100 g capacity, was used todisperse the MWCNTs in molten Nylon 66. Using a rotor/barrel gapdistance of about ⅓ inch, efficient mixing of the two components wasaccomplished through the high-shear stresses developed from therotational motion of the rotor inside the barrel. To prevent degradationduring processing, Argon gas was introduced into the mixing chamber at aflow rate of 0.244 Cft/hour.

To locate the linear viscosity region (LVR) of Nylon 66, a stress-sweeptest was performed at 277° C. using a R2000 rheometer, with a frequencyof 1 Hz and 10 points per decade (log mode). FIG. 3 shows that the (LVR)occurs at about 0.4% strain. To examine the behavior of the polymer atthis processing temperature, a frequency-sweep test was performed. FIG.4 shows curves for G′ (Pa), G″ (Pa) and Delta (degrees) vs. angularfrequency (rad/sec). This data is converted to viscosity (Pa·s) vs.shear rate (1/sec) in FIG. 5, which shows that the viscosity of Nylondecreases as the shear rate increases.

These data indicate that to process a CNT-reinforced Nylon 66 compositein the high-shear mixer, a mixing temperature of 277° C. (i.e. 10° C.above the melting point of Nylon 66) is required to yield adequateviscosity and shear rate. A mixing temperature of 10° C. above thepolymer melting point is considered to be a minimum for nylon 66, andwould be expected to be different for other polymers.

Example 2 Processing of Composites

Loosely agglomerated MWCNT powder was cold pressed in a Carver press,using a pressure of 4.5 metric tons and holding time of 5 minutes toproduce a compacted MWCNT. After pressing, the compact was broken upinto small pieces and vacuum dried. Following degassing, the now muchdenser CNT powder was introduced into the mixing unit and dispersed inthe Nylon 66 melt.

When the front and back sections of the mixing unit reach 10° C. abovethe 267° C. melting temperature of Nylon 66: 1) rotor speed was raisedgradually up to 50 rpm in 10 minutes, and held at this speed for anadditional 10 minutes; 2) 30 g of Nylon 66 was gradually fed into themixer and melted; 3) small pieces of cold-pressed CNT powder were addedto the molten polymer while ensuring good mixing; and 4) after feedingthe desired amount of CNT powder (8 g) and Nylon 66 (92 g) into themixer, the mixing parameters were fixed at 50 rpm to maintain viscosityas low as possible. To stabilize the mixing parameters, mixing speed wasraised to about 75 rpm, where it was held for 6 min to complete themixing process. Thereafter, the mixing speed was reduced gradually untilthe system automatically shut down due to the rapid increase inviscosity.

Incremental additions of CNTs to the molten Nylon are necessary toproduce a composite that contains a high fraction of CNTs. It takesabout 45 min to ensure that mixing parameters remain as stable aspossible. The rapid increase in melt viscosity during mixing isattributed to chemical bonding between dispersed CNTs and Nylon polymermatrix. After completion of the mixing process, the composite material,now having a rubber-like consistency, was extracted from the barrel atthe mixing temperature. Upon cooling to ambient temperature, thematerial became hard and brittle. This is further evidence for chemicalbonding between dispersed CNTs and Nylon 66 matrix.

Larger samples of CNT-reinforced Nylon can be prepared using anintegrated high shear mixing and injection molding apparatus. ASTMstandard test bars can be fabricated and evaluated for mechanicalproperties. Preliminary tests performed on small samples indicatesignificant improvements in stiffness and strength.

Example 3 Characteristics of Composites

FIGS. 6(a) through 6(d) show SEM images of a cryogenically-fracturedsurface of CNT-reinforced Nylon composite. The low-magnification imageshows a banded structure composed of alternating regions of slightlydifferent elevations. Interestingly, the low-elevation regions showevidence for pull-out of CNTs in the composite, but the high elevationregions do not, indicating that in these regions the fracture path cutsright through the CNTs. Even so, it is apparent that the high-shearmixing process has efficiently dispersed the original CNT agglomerates,forming a uniform distribution of CNTs in a Nylon matrix. See inparticular FIG. 6(b).

FIGS. 7(a) and 7(b) show representative TEM images of thin-tapered edgesof cryogenically-milled particles. These thin edges are difficult tofind, since only a tiny fraction of particles have edges that are thinenough to allow transmission of the electron beam. In FIG. 7(a), asingle MWCNT is in intimate contact with the Nylon 66 matrix, which isindicative of good adhesive bonding; however no covalent bonding isobserved for a MWCNT having no broken ends in a Nylon 66 matrix. Incontrast, whenever a MWCNT terminates in the field of view,dark-contrasting regions are observed. An example is shown in FIG. 7(b),which is interpreted to be evidence for the presence of crystallineNylon 66, which is denser than the surrounding amorphous matrix, thecrystallization of the polymer being induced by covalent bonding of theMWCNT to the polymer. See also FIGS. 9(a) to 9(c). Further, the TEMobservations revealed significant reduction in the length of CNTs.

FIGS. 8(a) through 8(f) show the DSC curves of composites which aremixtures of Nylon 66 and different percentages of long carbon nanotubes,from 1% to 6%, prepared by processing according to the disclosure. Theywere prepared at normal nylon process temperatures (about 300° C.) in ahigh shear batch mixer modified as described below, for 20 minutes.Under high shear mixing the long aspect fibers are broken, andcovalently bond polymer to the ends of the fibers. The figures displaythe normal melt peak and crystallization temperature for Nylon 66 itself(no carbon nanotubes covalently bound; see also FIG. 2), and a secondpeak having a higher recrystallization temperature for the polymercovalently bonded to CNTs. The latter peak successively increases as thepercentage of CNTs in the composition increases. A dramatic differenceoccurs between 5% and 6% CNTs. By 6% CNTs, the higher melt temperatureof the covalent adduct has taken over, with a new crystal form beingindicated. Overall there is about a 50-degree shift in melting andrecrystallization points of Nylon 66 as the carbon nanotubeconcentration is varied from 1% to 6%, with breakage of the carbonnanotubes in situ. This has not been observed or reported previously.

FIGS. 9(a) through 9(c) display TEM images of a MWCNT with freshlyfractured ends where bonding is observed with a Nylon 66 matrix. Alsosee FIG. 7(b). The pictures show a high density of polymer at the brokenends of the CNTs, indicating covalent bonding between the nanotubes andthe Nylon 66 polymer, vide supra. In contrast FIG. 7(a) displays a TEMimage showing that for a MWCNT having no broken ends, no bonding isobserved to the Nylon 66 matrix. This indicates that covalent bondinghas occurred between the nanotube fractured ends and the Nylon 66polymer when processed according to the present disclosure, which hasnot been previously observed or reported.

Such observations indicate that crystallization of Nylon 66 can beinitiated during high-shear mixing whenever CNTs experience fracture,thus exposing many dangling and reactive orbitals (free radicals) tobond with the molten polymer. This happens at a temperature above themelting point of Nylon 66, which is taken to be evidence for strongcovalent bonding between freshly-fractured ends of MWCNTs and moltenNylon 66.

Example 4

Continuous carbon fiber (CF) was cut to 1 meter lengths and fed directlyinto the hopper of a high, uniform shear injection molding machine withpolyetheretherketone (PEEK) in concentrations of 0, 10, 20, and 30 wt. %CF in PEEK. The CF fractured during high shear melt-processing withinmolten PEEK in accordance with an embodiment of the method of thepresent invention. Typically, CF is chopped to lengths ranging from 3 to10 mm prior to melt-processing. Using the present high shear processingmethod and continuous CF, there is an opportunity for fiber fracture tooccur while the fiber is surrounded by molten polymer, resulting indangling orbitals on the fiber ends available for covalent bonding withthe molten polymer. Primary covalent bonds between CF ends and thepolymer provide efficient load transfer, increased mechanical propertiesand high energy absorption capability. The composite morphology ispresented using field emission scanning electron microscopy andindicates very good fiber dispersion and distribution (see FIG. 10).Flexural properties were determined according to ASTM D790 and indicatesignificant increases in flexural modulus and strength. Izod impactresistance was determined according to ASTM D256 on notched specimenswith complete fractures and indicate a significant increase in impactresistance with increasing CF concentration. (See FIGS. 11(a)-(d)).Typically, fiber-reinforced thermoplastic composites suffer from lowerimpact resistance than the polymer alone. For example, the PEEKmanufacturer makes a 30 wt. % CF reinforced PEEK using chopped CF, andthey state that the Izod impact resistance decreases from 91 J/m to 69J/m for PEEK and 30 wt. % chopped CF in PEEK, respectively.

The foregoing examples and description of the preferred embodimentsshould be taken as illustrating, rather than as limiting the presentinvention as defined by the claims. As will be readily appreciated,numerous variations and combinations of the features set forth above canbe utilized without departing from the present invention as set forth inthe claims. Such variations are not regarded as a departure from thespirit and scope of the invention, and all such variations are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for forming a carbon fiber-reinforcedpolymer matrix composite, comprising: (a) distributing carbon fibersinto a molten carbon-containing polymer phase comprising one or moremolten carbon-containing polymers; (b) breaking or cutting the carbonfibers in the presence of the molten carbon-containing polymer phase by(i) applying a succession of shear strain events to the moltencarbon-containing polymer phase so that the molten carbon-containingpolymer phase breaks the carbon fibers, or (ii) mechanically breaking orcutting the carbon fibers in the presence of the moltencarbon-containing polymer phase, thereby producing new fiber ends on thecarbon fibers comprising free radicals that react with and directlycross-link by covalent bonding the one or more carbon-containingpolymers; and (c) thoroughly mixing the broken or cut carbon fibers withthe molten carbon-containing polymer phase, wherein the carbon fibersare selected from the group consisting of single-walled carbonnanotubes, multi-walled carbon nanotubes, carbon nanofibers, andmicron-sized carbon fibers.
 2. The method of claim 1, wherein at leastone of the one or more carbon-containing polymers contains one or moredouble bonds or one or more tertiary carbons.
 3. The method of claim 1,wherein the molten carbon-containing polymer phase comprises a nylon. 4.The method of claim 3, wherein the nylon is nylon
 66. 5. The method ofclaim 1, wherein graphite microparticles are distributed into the moltencarbon-containing polymer phase and mechanically exfoliated in additionto the carbon fibers.
 6. A method for forming a high-strength carbonfiber-reinforced polymer matrix composite, comprising: (a) forming thecomposite of claim 1 into cross-linked polymer particles; and (b)distributing the cross-linked polymer particles into a non-cross-linkedmolten host matrix polymer.
 7. A carbon fiber-reinforced polymer matrixcomposite prepared according to the method of claim 1, consistingessentially of carbon fibers distributed into a molten carbon-containingpolymer phase comprising one or more molten carbon-containing polymers,wherein said polymers are cross-linked by direct covalent bonds to theends of said carbon fibers, and said composite optionally furtherincludes mechanically exfoliated graphene distributed therein.
 8. Thecarbon fiber-reinforced polymer matrix composite of claim 7, wherein thepolymer is nylon
 66. 9. A high strength carbon fiber-reinforced polymermatrix composite prepared according to the method of claim 6, whereinthe cross-linked polymer particles consist essentially of carbon fibersdistributed into a carbon-containing polymer phase comprising one ormore carbon-containing polymers, wherein said polymers are cross-linkedby direct covalent bonds to the ends of said carbon fibers, and saidcomposite optionally further includes mechanically exfoliated graphenedistributed therein.
 10. The method of claim 1 or 6, wherein thecomposite shows improved stiffness and strength versus a compositelacking covalent bonding between carbon fibers and polymer.
 11. Themethod of claim 1 or 6, wherein the composite shows improved impactenergy absorption versus a composite lacking covalent bonding betweencarbon fibers and polymer.
 12. The carbon fiber-reinforced polymermatrix composite of any one of claims 6 to 9, wherein the compositeshows improved stiffness and strength versus a composite lackingcovalent bonding between carbon fibers and polymer.
 13. The carbonfiber-reinforced polymer matrix composite of any one of claims 7 to 9,wherein the composite shows improved impact energy absorption versus acomposite lacking covalent bonding between carbon fibers and polymer.14. A polymer composite consisting essentially of polymer chainsinter-molecularly and directly cross-linked by broken carbon fibers,wherein said polymer-fiber cross-links consist essentially of directcovalent bonds to the ends of said carbon fibers, and said compositeoptionally further includes mechanically exfoliated graphene distributedtherein.
 15. An automotive, aircraft or aerospace part formed from thecomposite of claim
 14. 16. The part of claim 15, wherein the part is anengine part.
 17. Carbon fiber cross-linked polymer particles formed fromthe composite of claim
 14. 18. A polymer composition comprising a hostthermoplastic polymer and the carbon fiber cross-linked polymerparticles of claim 17 dispersed therein.
 19. An automotive, aircraft oraerospace part formed from the polymer composition of claim
 18. 20. Themethod of claim 1, wherein the breaking of carbon fibers occurs throughhigh shear melt processing.
 21. The method of claim 1, wherein thepolymer is selected from the group consisting of polyetherketones (PEK),polyphenylene sulfides (PPS), polyethylene sulfide (PES),polyetherimides (PEI), polyvinylidene fluoride (PVDF), polysulfones(PSU), polycarbonates (PC), polyphenylene ethers, aromatic thermoplasticpolyesters, aromatic polysulfones, thermoplastic polyimides, liquidcrystal polymers, thermosplastic elastomers, polyethylene,polypropylene, polystyrene (PS), acrylics, ultra-high-molecular-weightpolyethylene (UHMWPE), polytetrafluoroethylene (PTFE/Teflon®),polyamides (PA), polyphenylene oxide (PPO), polyoxy methylene plastic(POM/Acetal), polyarylether-ketones, polyvinylchloride (PVC), andmixtures thereof.